1. Field of the Invention
The present invention relates to a process for preparing dithiinetetracarboximides by reaction of succinic monoamides with thionyl chloride, with continuous performance of at least one of the process steps.
2. Description Of Related Art
Dithiinetetracarboximides as such are already known. It is likewise known that these dithiinetetracarboximides can be used as anthelmintics to counteract internal parasites in animals, especially nematodes, and have insecticidal action (cf. U.S. Pat. No. 3,364,229). It is also known that particular dithiinetetracarboximides have antibacterial action and have a certain effect against organisms which cause human mycoses (cf. Il Farmaco 2005, 60, 944-947). It is additionally known that dithiinetetracarboximides can be used as pigments in electrophotographic photoreceptors or as dyes in coatings and polymers (cf. JP-A 10-251265, PL-B 143804).
Dithiinetetracarboximides of the formula (I)
in which                R1 and R2 are the same or different and are each hydrogen, optionally mono- or poly-halogen-, —OR3—, —COR4—substituted C1-C8-alkyl, optionally mono- or poly-halogen-, —C1-C4-alkyl- or —C1-C4-haloalkyl-substituted C3-C7-cycloalkyl, in each case optionally mono- or poly-halogen-, —C1-C4-alkyl-, —C1-C4-haloalkyl-, —COR4— or -sulphonylamino-substituted aryl or aryl-(C1-C4-alkyl),        R3 is hydrogen, C1-C4-alkyl, C1-C4-alkylcarbonyl or optionally mono- or poly-halogen-, —C1-C4-alkyl- or —C1-C4-haloalkyl-substituted aryl,        R4 is hydroxyl, C1-C4-alkyl or C1-C4-alkoxy, can be prepared in various known ways.        
For example, in one process (cf. U.S. Pat. No. 3,364,229; Chem. Ber. 1967, 100, 1559-1570), in a first stage, dichloromaleic anhydride of the formula (II) is reacted with an amine of the formula (III), optionally in the presence of a diluent. Subsequently, the dichlormaleimides of the formula (IV) thus obtained are reacted with a sulphur compound (e.g. hydrogen sulphide or thiourea). The preparation of the dithiinetetracarboximides of the formula (I) by this process can be illustrated by the following scheme:

This process has the disadvantage that, for example, the handling of hydrogen sulphide gas, which is highly toxic, is technically very difficult and inconvenient. In the case of use of thiourea, apart from the target product, unwanted by-products are obtained, these being removable only with very great difficulty and worsening the yields achievable (cf. J. Heterocycl. Chem. 1988, 25, 901-906).
In a further process which has become known (cf. Synthetic Communications 2006, 36, 3591-3597), in a first stage, succinic anhydride of the formula (V) is reacted with an amine of the formula (III), optionally in the presence of a diluent. Subsequently, the succinic monoamides of the formula (VI) thus obtained are reacted with a large excess of thionyl chloride in the presence of dioxane as a diluent at room temperature for 6 hours, the dithiinetetracarboximides of the formula (I) finally being obtained in a sequence of numerous reaction steps. The dithiinetetracarboximides are either isolated directly from the reaction mixture or by filtration after addition of water. According to the reaction conditions (diluent) and nature of the R radicals, under some circumstances, the dithiinediisoimides of the formula (VII) can be isolated before they are converted to the dithiinetetracarboximides of the formula (I). This method for preparing the dithiinetetracarboximides of the formula (I) can be illustrated by the following scheme:

Disadvantages of this process are the long reaction time and the result that either the yields obtained generally do not exceed about 30-40% of theory, or else the purities of the isolated products are inadequate. Another disadvantage in the case of aqueous workup of the reaction mixture is that large amounts of thionyl chloride are destroyed; the gases which arise (SO2 and HCl) have to be disposed of. Likewise disadvantageous is the fact that, as experience has shown, the product is not obtained in one fraction. Instead, it is frequently the case that, after a first product isolation by filtration further product precipitates out of the filtrate after standing for a prolonged period (for example overnight), and this has to be isolated by filtration again. Sometimes, this operation has to be performed once again. This way of working is very inconvenient and time-consuming.
It is additionally known that dithiinetetracarboximides are obtained by dissolving N-substituted succinamides in dry 1,4-dioxane and then admixed with thionyl chloride. Subsequently, the reaction mixture is heated and the solution is concentrated in vacuo and separated and purified by means of column chromatography (cf. J. Heterocycl. Chem. 2010, 47, 188-193).
It is additionally known that dithiinetetracarboximides are obtained by admixing N-substituted succinamides with thionyl chloride, optionally in the presence of an inert diluent. Subsequently, the excess thionyl chloride is distilled off, and the remaining reaction mixture is heated in the presence of water and optionally in the presence of an inert diluent (cf. WO 2011/128263).
The prior art processes, however, have the following disadvantages:                a) the low space-time yield,        b) the large excess of thionyl chloride based on the succinic monoamides, which requires a high level of technical complexity in the workup of the reaction output and recovery of the unconverted thionyl chloride and hence high capital and energy costs,        c) the offgas flow released in the reaction does not run homogeneously and hence makes it difficult to implement a technically simple and economic preparation process possibly including the simultaneous reprocessing of the offgas stream for the purpose of reutilization.        